This invention relates generally to color imaging and more particularly to the use of plural exposure and development steps for such purposes.
It is common practice to add information to the face of a document or to highlight certain portions of it by underlining. It is also common to delete portions of the document either by crossing out information or by covering it with a blank piece of paper. As will be appreciated, writing data or underlining on the document spoils the original document while writing data or underlining on the copies requires much labor when many copies are required. Moreover, it is sometimes difficult to write on copies due to the impregnation of the paper substrate with silicone oil used in the fusing of xerographic images to the substrate.
Recent developments in imaging systems have obviated the foregoing problems by the provision of methods and apparatus to reproduce altered copies of an original document. This, recent innovations in printing machines provide for reproducing portions of a document in one color and other portions thereof in a different color.
One method of printing in different colors is to uniformly charge a charge retentive surface and then optically expose the surface to information to be reproduced in one color. This information is rendered visible using marking particles followed by the recharging of the charge retentive prior to a second exposure and development.
U.S. Pat. No. 4,791,452 relates to two-color imaging apparatus wherein a first latent image is formed on a uniformly charged, charge retentive surface and developed with toner particles. The charge retentive surface containing a first developed or toned image and undeveloped or untoned background areas is then recharged prior to optically exposing the surface to form a second latent electrostatic image thereon. The recharging step is intended to provide a uniformly charged imaging surface prior to effecting a second exposure.
U.S. Pat. No. 4,819,028 discloses an electrophotographic recording apparatus capable of forming a clear multicolor image including a first visible image of a first color and a second visible image of a second color on a photoconductive drum. The electrophotographic recording apparatus is provided with a conventional charger unit and a second charger unit for charging the surface of the photoconductive drum after the first visible image is formed thereon so as to increase the surface potential of the photoconductive drum to prevent the first visible image from being mixed with a second color and scratched off from the surface of the photoconductive drum by a second developing unit.
U.S. Pat. No. 4,660,961 discloses a copying apparatus of the electrostatic type which enables two images to be synthesized on one surface of a copying paper using original positive image sources without preparing negative images sources prior to the copying process. The copying apparatus can also synthesize a plurality of images in different colors on a single sheet of paper.
U.S. Pat. No. 4,761,669 relates to creating two-color images. A first image is formed using the conventional xerographic process. Thus, a charge retentive surface is uniformly charged followed by light exposure to form a latent electrostatic image on the surface. The latent image is then developed. A corona generator device is utilized to erase the latent electrostatic image and increase the net charge of the first developed image to tack it to the surface electrostatically. This patent proposes the use of an erase lamp, if necessary, to help neutralize the first electrostatic image. A second electrostatic image is created using an ion projection device. The ion image is developed using a second developer of a different color.
U.S. Pat. No. 4,033,688 discloses a color copying apparatus which utilizes a light-lens scanning device for creating plural color images. This patent discloses multiple charge/expose/develop steps.
U.S. Pat. No. 4,833,503 discloses a multi-color printer wherein a recharging step is employed following the development of a first image. This recharging step, according to the patent is used to enhance uniformity of the photoreceptor potential, i.e. neutralize the potential of the previous image.
U.S. Pat. No. 4,660,059 discloses an ionographic printer. A first ion imaging device forms a first image on the charge retentive surface which is developed using toner particles. The charge pattern forming the developed image is neutralized prior to the formation of a second ion image.
U.S. patent application Ser. No. 856,311 filed on Mar. 23, 1992 and assigned to the same assignee as the instant application discloses a printing system wherein charged area images and discharged area images are created, the former being formed first and the latter being proceeded by a recharging of the imaging surface.
A number of commercial printers employ the charge/expose/develop/recharge imaging process. For example, the Konica 9028, a multi-pass color printer forms a single color image for each pass. Each such pass utilizes a recharge step following development of each color image. The Panasonic FPC1 machine, like the Konica machine is a multi-pass color device. In addition to a recharge step the FPC1 machine employs an AC corona discharge device prior to recharge.
In the method of creating multi-color images using a conventional charge/expose/develop process as illustrated in the patents discussed above, voltage non-uniformity between developed (toned) and non-developed (untoned) areas on the charge retentive occurs. This non-uniformity in potential causes undesirable edge effects. The edge effect phenomena results in development of the edges of an image of one color with the marking particles of a second color.